Polyurethanes (PU) are known, which are characterized by the presence, in their structure, of polyoxyperfluoroalkylene blocks deriving from the uses of perfluoropolyethers having hydroxylated end groups.
Products of this type are described in particular in Italian Patent No. 903,446. They are characterized by a Tg lower than -80.degree. C., wherefore they retain flexibility and elastic properties even at very low temperatures.
The structure of these materials is free from stiff segments. To have consistency, it is necessary to impart to them a three-dimension lattice by cross-linking them either with three-functional chemical agents or through the formation of allophenate or isocyanurate.
However, the materials so obtained do not possess an optimal combination of mechanical characteristics regarding hardness, tensile strength, modulus of elasticity and elongation. In particular, the hardness values are lower than 50 Shore A. The tensile strength values are generally lower than 10 kg/cm.sup.2.
A substantial improvement in the aggregate of mechanical properties has been obtained using the introduction of encumbering blocks consisting of aromatic or cycloaliphatic diols as is illustrated in applicants prior European patent application no. 192,190. However, the obtained products do not exhibit an optimal combination of properties. In particular, the fluorinated polyurethanes obtained according to the process are characterized by tensile strength values which generally do not exceed 25 kg cm.sup.2. Furthermore, the presence of aromatic diols in the structure is a limitation for the use thereof at high temperatures.
In these products, the modulus of elasticity undergoes considerable variations as a function of temperature, so that it sinks to very low values at temperatures close the melting temperatures.
The above references are hereby incorporated by reference.